Messages in this thread |  | | Date | Mon, 8 Dec 2025 18:57:25 +0000 | | From | David Laight <> | | Subject | Re: [PATCH] mm: avoid use of BIT() macro for initialising VMA flags |
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On Mon, 8 Dec 2025 16:42:43 +0000 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
> On Sat, Dec 06, 2025 at 04:43:57PM +0000, Lorenzo Stoakes wrote: > > On Fri, Dec 05, 2025 at 09:34:49PM +0000, David Laight wrote: > > > On Fri, 5 Dec 2025 19:18:56 +0000 > > > Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote: > > > > > > > On Fri, Dec 05, 2025 at 06:43:42PM +0000, David Laight wrote: > > > > > On Fri, 5 Dec 2025 17:50:37 +0000 > > > > > Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote: > > > > > > > > > > > Commit 2b6a3f061f11 ("mm: declare VMA flags by bit") significantly changed > > > > > > how VMA flags are declared, utilising an enum of VMA bit values and > > > > > > ifdef-fery VM_xxx flag declarations via macro. > > > > > > > > > > > > As part of this change, it uses INIT_VM_FLAG() to define VM_xxx flags from > > > > > > the newly introduced VMA bit numbers. > > > > > > > > > > > > However, use of this macro results in apparently unfortunate macro > > > > > > expansion and resulted in a performance degradation.This appears to be due > > > > > > to the (__force int), which is required for the sparse typechecking to > > > > > > work. > > > > > > > > > > Does sparse complain if you just add 0? As in: > > > > > #define INIT_VM_FLAG(name) BIT(VMA_ ## name ## _BIT + 0u) > > > > > > > > > > That should change the type without affecting what BIT() expands to. > > > > > > > > Thanks, checked that and unfortunately that doesn't satisfy sparse :) > > > > > > > > I don't think it's too crazy to use 1UL << here, just very frustrating (TM) > > > > that this is an issue. > > > > > > I might use some of my copious spare time (ha) to see why BIT() fails. > > > I bet it is just too complex for its own good. > > > Personally I'm fine with both explicit (1ul << n) and hex constants. > > > The latter are definitely most useful if you ever look at hexdumps. > > > > Thanks :) yeah I just didn't want to go down that rabbit hole myself as I seemed > > to have the answer and wanted to get it fixed, but obviously am quite curious as > > to what on earth is causing that. > > I did wonder about _calc_vm_trans(), given the 'interesting' stuff it does. > > Maybe I should fiddle with that and see...
Hmmm... /* * Optimisation macro. It is equivalent to: * (x & bit1) ? bit2 : 0 * but this version is faster. * ("bit1" and "bit2" must be single bits) */ #define _calc_vm_trans(x, bit1, bit2) \ ((!(bit1) || !(bit2)) ? 0 : \ ((bit1) <= (bit2) ? ((x) & (bit1)) * ((bit2) / (bit1)) \ : ((x) & (bit1)) / ((bit1) / (bit2))))
The comment fails to mention it is only sane for constants. If nothing else 9 expansions of BIT() are going to generate a very long line.
For starters make it a statement expression and use __auto_type _bit1 = bit1. Then add a check for both _bit1 and _bit2 being constants.
It is also worth checking the compiler doesn't do it for you. Looks like gcc 7.1 onwards generate the 'optimised' code.
https://godbolt.org/z/EGGE56E3r
David
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